The large quantities of sytrene required by industry are derived by dehydrogenation of ethyl benzene, supplied in part by fractional distillation of hydrocarbon fractions of eight carbon atom aromatics such as those separated from catalytic reformates. The other primary source of ethyl benzene is from alkyalation of benzene, usually with ethylene. The commercial alkylation plants presently in operation employ catalysts of the Friedel-Crafts type, usually aluminum chloride. The liquid catalyst of that type in a stirred reactor makes it possible to remove the heat of reaction by conventional techniques.
The Friedel-Crafts catalysts are highly corrosive and require that reactors, heat exchangers and other auxiliaries by fabricated from expensive corrosion-resistant materials. The spent catalyst and other waste from such plants present a troublesome pollution problem. In addition, the reactants must be very pure to avoid undesirable by-products. In an adaptation of Friedel-Crafts catalyst to the use of catalytic cracking tail gas as source of ethylene for alkylation of benzene, all possibly reactive components other than ethylene must be scrupulously excluded. Hydrogen sulfide, carbon dioxide and water normally present in such tail gas are removable at little cost by caustic scrubbing to absorb the acidic gases hydrogen sulfide and carbon dioxide and by condensation of the water contained in the tail gas or picked up during scrubbing. Carbon monoxide requires much more expensive technique, but obviously must be removed before the gas is brought into contact with a Friedel-Crafts catalyst.
It will be immediately apparent that much of the disadvantage of Friedel-Crafts catalyst will be avoided by the use of a solid heterogeneous catalyst. Many solid porous catalysts having acid character have been shown to be active for the reaction of ethylene with benzene to synthesize ethyl benzene. Typically, the charge to such reaction will be a mixture in which the mole ratio of benzene to ethylene is high enough to suppress the formation of polyethyl benzenes and such amounts of these by-products as may be formed are subjected to transalkylation with benzene, either in the alkylation reactor or in a separate vessel. The acid catalysts are effective for promotion of polymerization. Some alkyl benzenes having larger side chains than ethyl can be found in the product. In addition the activity of the catalyst declines very rapidly, possibly due to formation of high molecular weight compounds which remain on the catalytic surfaces.
It has been shown that zeolites in the nature of zeolite ZSM-5 show high activity and selectivity for alkylation of benzene with ethylene and that catalysts of this type in the acid form remain active for unusually long periods between regenerations to burn off carbonaceous deposits which render the catalyst ineffective. Good discussion of acid zeolite ZSM-5 for this purpose is provided in U.S. Pat. No. 3,751,506, granted Aug. 7, 1973 on an application of George T. Burress. That patent proposes control of the exothermic heat of reaction by conducting the reaction is a series of reactors with intermediate cooling and addition of ethylene between stages. Note is there made that the course of the reaction may be affected by diluents and examples are given in which nitrogen is added in an amount equal to 0.5 mole per mole of ethylene.